for people who doesnt read manga and just wants to enjoy watching an anime series for the first time. Some anime's opening themes can be such a spoiler because they show all of the character on the opening theme that we wont be surprised anymore when the villains and the other supporting characters came into scene.....

perfect example is tales of the abyss... ive already seen the main antagonist on the opening theme so there is no use whether they darkened his face on the episodes (to give that mysterious vibe) and he makes a big entrance in the future episodes.

The existence of characters aren't exactly a secret. If the opening included say...the main character getting stabbed or making out with somebody, that would be a spoiler. Like in harem anime, when there is a shot of the guy and a girl and they "almost" kiss or something you know she is going to win.

How would you know who is a villain in the opening if you haven't seen the show? I don't think they are spoiling so much as they are teasing. Presumably they show you those characters to show you that the plot will grow deeper and relations will get more complicated as it progresses, and that other factors will come into play. That and its a little bit of a hint for people who don't know what's really going on about which characters are important.

The Clannad opening. If Fuko didn't show up, imagine how many fans would just scream, yell, cry, jump, etc. when they see Fuko actually show up. Since we now know Fuko will eventually at some point appear, it won't leave as much of an impact. The opening song is too good to skip.